1. Field of the Invention
This invention concerns an alumina-based composite sintered material which is a sintered material comprising alumina as a main ingredient and, more particularly, it relates to a wear resistant member such as a cutting tool having excellent wear resistance and chipping resistance, as well as a method of manufacturing an alumina-based composite sintered material.
2. Description of the Related Art
Heretofore, alumina-based composite sintered materials have been used generally as wear resistant members (wear resisting members) including cutting tools since they have excellent mechanical properties. Further, various improvements have been attempted in recent years by compounding with other ingredients for further improving performance.
For example, Japanese Patent Laid-Open No. 104943/1997 proposes a technique of compounding zirconia (ZrO2), thereby suppressing development of cracks to increase toughness and improve the chipping resistance.
However, since zirconia has low hardness and poor heat thermal impact resistance, it is not yet satisfactory in view of wear resistance and heat resistance.
Further, Japanese Patent No. 2720093 proposes a technique of compounding titanium carbide (TiC) of high hardness with needle alumina (Al2O3) to increase toughness while keeping hardness to some extent, thereby improving the chipping resistance in addition to providing excellent wear resistance.
However, needle alumina not only is disadvantageous because it is expensive, but also tends to form gaps at its periphery such that incorporation of needle alumina can cause defects.
Further, since titanium carbide is less sinterable, the material has to be densified by sintering at high temperature or applying hot pressing. However, this results in a problem that the entire structure grows into coarse grains which lowers hardness and lacks wear resistance when sintered at high temperature, or the shape is restricted when sintered by hot pressing.
Further, a technique of re-sintering an HIP sintered material in an N2 atmosphere to make a value N/(C+N) maximum at the surface and decrease the value from the surface to the interior is disclosed in Japanese Patent Laid-Open No. 208304/1993. However, when the HIP sintered material is re-sintered, pores closed during the HIP process form open pores again and the structure at the surface tends to grow into coarse grains which lowers the strength. Further, since TiN has a lower hardness and tends to be poor in wear resistance compared with TiC, it is necessary to control the nitridation amount at the surface in order to maintain the wear resistance. However, the HIP sintered material is substantially densified such that control of the amount of N2 introduced at the surface by re-sintering is extremely difficult.
This invention has been made in view of the foregoing problems of the prior art. Therefore, an object of the present invention is to reduce the defects of alumina-based composite sintered materials to obtain an alumina-based sintered material and a wear resistant member not only excellent in wear resistance but also excellent in chipping resistance, as well as a method of manufacturing an alumina-based composite sintered material.
Accordingly, the present invention provides an alumina-based composite sintered material comprising alumina as a main ingredient and containing one or more carbonitridation products of groups IVa, Va and VIa of the periodic table and/or carbonitridation products of two or more solid solutions of groups IVa, Va and VIa of the periodic table, wherein the content of nitrogen solid solute in the carbonitridation product increases from the interior to the surface of the sintered material, and the Vickers hardness at the surface of the sintered material is 19.5 GPa or more.
It is considered, for example, as shown in FIG. 1(a) of the accompanying drawings, that titanium carbide (TiC) grains intrude between alumina grains in the alumina-based composite sintered material according to this invention. Nitrogen (N) is dissolved as a solid solute in titanium carbide to form a solid solution (Ti(C, N)) of a carbonitridation product. The content of nitrogen solid solute in titanium carbide is expressed by the size of the annulus around the central circle representing TiC.
As can also be seen from FIG. 1(a), the content of nitrogen solid solute in the carbide increases from the interior to the surface of the sintered material. On the other hand, in the existent product shown in FIG. 1(b), the content of nitrogen solid solute in the carbide is substantially identical between the interior and the surface of the sintered material.
As described above, since the content of nitrogen solid solute in the carbonitridation product increases from the interior to the surface of the sintered material, that is, the content of nitrogen solid solute in the carbonitridation product is greater in the surface than in the interior of the sintered material in this invention, sinterability is high to enable densification at a low temperature. Therefore, a sintered material of high hardness and high strength having a fine structure, that is, an alumina-based composite sintered material having excellent wear resistance and chipping resistance, is obtained.
Further, since the Vickers hardness at the surface of the sintered material is 19.5 GPa or more due to the distribution and content of nitrogen solid solute described above, this invention can provide sufficient wear resistance, for example, as wear resistant cutting tools.
The alumina-based composite sintered material referred to herein is a sintered material comprising alumina as a main ingredient (for example, 60% by weight or more), compounded with the addition of other ingredients. Further, the load used for measuring the Vickers hardness is 1.0 kgw.
One or more carbonitridation products of groups IVa, Va and VIa of the periodic table can include, for example, Ti(C, N), Zr(C, N) for group IVa, V(C, N), Nb(C, N), Ta(C, N) for group Va and VIa Cr3(C, N) for group VIa.
Further, at least two carbonitridation products of solid solutions of groups IVa, Va and VIa of the periodic table can include, for example, (Ti, W)(C, N) and (W, Ta, Nb)(C, N).
The alumina-based composite sintered material according to this invention contains at least one carbonitridation product of groups IVa, Va and VIa of the periodic table and/or at least two carbonitridation products of solid solutions of groups IVa, Va and VIa of the periodic table, and it may contain both carbonitridation products.
Preferably the element belonging to groups IVa, Va and VIa of the periodic table is one or more of Ti, V and Zr.
This exemplifies metal elements contained in carbonitridation products of solid solutions of groups IVa, Va and VIa of the periodic table.
By using metal elements, an alumina-based composite sintered material having excellent properties described above can be obtained.
One or more carbonitridation products of Ti, V and Zr can include, for example, Ti(C, N), V(C, N) and Zr(C, N). Further, two or more carbonitridation products of solid solutions of Ti, V and Zr can include, for example, (Ti, V)(C, N) and (Ti, Zr)(C, N).
Preferably, the content of the carbonitridation product in the alumina-based sintered material is 10 to 40% by weight.
The content of the carbonitridation product (based on the entire sintered material) is defined as 10 to 40% by weight. This is because particles of alumina as the main ingredient do not grow into coarse grains and do not lower the strength and the hardness of the sintered material when the content of the carbonitridation product is 10% by weight or more. Also, the sinterability is not lowered and densification at low temperature where the entire structure becomes fine is possible when the content of the carbonitridation product is 40% by weight or less.
Then, a more preferred effect is obtained within a range of 25 to 35% by weight.
The carbonitridation product comprises one or more carbonitridation product(s) of groups IVa, Va and VIa of the periodic table and/or two or more carbonitridation products of solid solutions of groups IVa, Va and VIa of the periodic table.
Preferably the content of nitrogen solid solute in the carbonitridation product is 40 atom % (at %) or less. The value of the nitrogen content in at % represents the proportion of carbon atoms that have been substituted by nitrogen atoms.
The content of nitrogen solid solute in the carbonitridation product is defined as 40 at % or less (naturally exceeding 0 at %), because the hardness of the carbonitridation product is not lowered and lowering of strength and hardness by the promotion of grain growth of the main ingredient alumina can be prevented to suppress growth into coarse grains at an amount of 40 at % or less.
Then, a more preferred effect is obtained within a range of 20 to 30 at %.
The effect by specifying the content of nitrogen solid solute is remarkably obtained at the surface of the alumina-based composite sintered material in contact with a workpiece, for example, in the case of using a cutting tool.
The carbonitridation product includes one or more carbonitridation products of groups IVa, Va and Via of the periodic table and/or two or more carbonitridation products of solid solutions of groups IVa, Va and Via of the periodic table.
Preferably a hard layer, formed by coating with a hard material, is provided on the surface of the sintered material.
TiC, TiN, TiCN and/or Al2O3 can be used as the hard material, for instance, to thereby further improve the wear resistance.
As the coating method, PVD (Physical Evaporation Deposition method) or CVD (Chemical Vapor Deposition method) can be used, for instance.
The invention further provides a wear resistant member comprising an alumina-based composite sintered material as defined above.
The wear resistant member according to this invention is composed of an alumina-based composite sintered material described above. Accordingly, it has excellent wear resistance and chipping resistance.
The invention further provides that the wear resistant member comprising the wear resistant material can be a cutting tool.
Accordingly, the cutting tool has excellent wear resistance and chipping resistance.
The wear resistant member can include, in addition to the cutting tool, for example, bearings, dies, press molds, chain saw blades and sliding members of engines.
According to another aspect, the invention provides a method of manufacturing an alumina-based composite sintered material comprising alumina as a main ingredient, wherein nitrogen is dissolved as a solid solute in carbides to form carbonitridation products in a powder molded product formed by compounding one or more carbides of groups IVa, Va and VIa of the periodic table with alumina as a main ingredient, while controlling an Ar/N2 gas mixing ratio and a temperature elevating rate upon sintering, whereby the content of nitrogen solid solute in the carbonitridation product is increased from the interior to the outside of the sintered material.
According to this invention, sinterability can be improved and the material can be densified at low temperature by further increasing the content of nitrogen solid solute near the surface of the sintered material while maintaining hardness by nitriding the surface of the starting carbide material. This can provide a sintered material of high hardness and high strength having a fine structure.
Accordingly, when used as the wear resistant member, excellent wear resistance and chipping resistance can be obtained.
An HIP (Hot Isostatic Pressing) treatment may optionally be applied for increasing density.
The present invention is based on the following findings. An Ar/N2 gas mixing ratio and a temperature elevation rate are defined within specified ranges upon sintering of a powder molded product comprising alumina as a main ingredient, compounded with carbides of groups IVa, Va and VIa of the periodic table. This dissolves nitrogen as a solid solute into the carbides to form carbonitridation products and increases the content of nitrogen solid solute in the carbonitridation product from the interior to the surface of the sintered material. It is considered that grain boundary diffusion of alumina and carbide is promoted as the content of nitrogen solid solute increases. Since this improves the sinterability to enable densification at low temperature, it is possible to obtain a sintered material of high hardness and high strength having a fine structure.
More specifically, by nitriding the surface of the starting carbide material, a sintered material of high hardness and high strength having a fine structure is obtained by further improving the sinterability to increase the density near the surface of the sintered material while maintaining the hardness of the starting carbide material. When used as a wear resistant member, excellent wear resistance and chipping resistance can be achieved.